Electric discharge apparatus



y 1940. K. H. KINGDON 2,209,819

ELECTRI C DI S CHARGE APPARATUS Filed July 29, L939 2 Sheets-Sheet 1Fig. I. 44 4! 42 ,3

J 45 a n a; a4 37 as Inventor: Kenneth H. Kin don, 4 m 17 (5,9

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July so, 1940. KlNGDON 2,209,819

ELECTRIC DISCHARGE APPARATUS Filed July 29, 1939 2 Sheets-Sheet 2Inventor: Kenneth H. Kin don.

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Patented July 30, 1940 UNITED STATES 2,209,819 I Emc'rmo DISCHARGEAPPARATUS Kenneth H. Kingdon, Schenectady,

N. Y., as-

signor to General Electric Company, a corporation of New YorkApplication July 29, 1939, Serial No. 287,341

4 Claims.

The present invention relates to improvements in "controllablearc-discharge apparatus and has for its primary objectthe provision ofapparatus of this character which is adapted for use at high voltages.

In connection with the attainment of the above-stated object, animportant feature of the invention consists in the inclusion in agridcontrolled arc-discharge apparatus of one or more intermediateelectrode elements between the control grid and the anode forsubdividing the voltage drop between them in such fashion as to lessenthe possibility of undesired breakdown.

Another important feature, ancillary to the above, involves the use ofmeans for maintaining a potential relationship between the variousintermediate electrodes of such character as to decrease the timeinterval which must elapse between the end of one conducting period ofthe apparatus and the instant when a dischargefavoring potential mayagain be applied to such apparatus without the immediate occurrence ofbreakdown.

The aspects of the invention which I desire to protect herein arepointed out in the appended claims. The invention itself, together withfurther objects and advantages thereof, may best be understood byreference to the following description taken in connection with thedrawings in which Fig. 1 comprises a sectional view of a dischargedevice embodying the invention, together with suitable circuitconnections for the same; Fig. 2 shows diagrammatically the circuitconnections of Fig. l, and Fig. 3 illustrates an inverter circuit inwhich the invention is applied.

For convenience in exposition the elements of the invention will firstbe described briefly without detailed reference to function. Theircooperative relationship then will be explained in connection with aparticular utilization circuit.

Referring particularly to Fig. 1, there is shown an elongated dischargeenvelope comprising, for example, a cylindrical metal portion lil whichis closed at its upper end by means of a transverse header I boltedthereto and which is joined at its lower end to a smallercathodecontaining portion l2. The part I! is insulated from the mainbody of the envelope by means of an insulator l3 and is united to thecylinder ID by means of a bolted joint, indicated at H. The lateralwalls of the envelope are cooled by means of a cooling coil l6 which isarranged in good thermal contact therewith. The envelope is alsoprovided with an exhaust duct ll adapted for connection with anevacuating system.

At the upper end of the envelope there is provided an anode l9 which issupported by means of a hollow current-supply conductor 20. Thisconductor is insulatingly joined to the envelope by means of aglass-to-metal seal indicated at 2 The cathode of the device may consistof a solid electron emitter or, as shown, of a pool of mercury 23 whichserves also as a source of ionizable vapor during the operation of theapparatus. (A fixed gas, such as argon, may also be utilized in somecases as an auxiliary ionizing medium.) Above the cathode thereisprovided a drip baflle 25 which has an annular troughlike peripheralportion adapted to receive mercury condensate running down the walls ofthe envelope and to prevent such condensate from streaming over thesurface of the insulator !3. Current may be supplied to the cathode bymeans of a heavy stud 21 secured to the bottom surface of the envelopepart l2. For initiating a cathode-spot on the mercury surface, there isprovided a make-alive electrode 28 of the immersion type; this beingconnected with a source of energizing potential (not shown) through alead-in conductor 29.

Within the envelope and spaced from the cathode 23 thereis arranged akeep-alive electrode 3| which is adapted to maintain a holding arc tothe cathode surface. This is provided with current through a lead-inconnection 32 which is protected from the effects of the arc stream bymeans of an insulating sheath 33.

Between the anode and cathode and in the vicinity of the former thereare provided a plurality of grid electrodes, numbered 35 to 38respectively. The grids 35 and 36 are so formed as substantially toenclose the anode and are supported and energized by means of conductivecylinders 40 and ti which respectively support them from metal lead-inrings indicated at 42 and 43. (The glass-to-metal seals by which therings 42 and 43 are insulatingly mounted are protected againstbombardment by means of a series of shielding sleeves 44.) The grids 31and 38 are cup-shaped and are insulatingly supported from the envelopeheader ll. They are respectively supplied with current through lead-inseals15 and 46. All of the grids are provided with perforations in thoseWalls thereof which are directed toward the cathode, these perforationsbeing of suificient size to permit passage of the discharge current.

In Fig. 1, and in another form in Fig. 2, there is shown a circuit withwhich the structure so far described may be employed to utilize theprinciples of my present invention. In this circuit, the anode l3 andthe cathode 23 are intended to provide between them an intermittentlyconducting discharge path. To this end they are shown as beingrespectively connected to circuit conductors 48 and 49. These conductorsin turn are to be connected to the terminals of an appropriate source ofalternating or direct voltage depending on the conditions of use of theapparatus.

In order to control the conductivity of the discharge path the grid 31is connected to one teminal of the secondary of a transformer 5! bymeans of which its potential relationship to the cathode 23 may becontrolled. It will be understood that as longas the grid 31 ismaintained negative with respect to the cathode, it will tend to preventthe occurrence of a discharge. On the other hand, when it is madepositive with respect to the cathode it will tend to favor such adischarge. Its operation may be controlled in a desired manner byconnecting the primary of the transformer 5| to a source of controlpotential (not shown) whose periodicity and phase are properly relatedto those of the anode-cathode potential. Transient influences, whichmight otherwise tend to produce sporadic firing of the tube, can besubstantially eliminated by the use of a condenser 53 shunted across thetransformer 5|.

To facilitate the functioning of the control grid 31 there is providedin connection therewith an additional grid 33. The two'grids referred toare connected to a common terminal of the transformer 5| throughresistance elements 54 and 55 which are of different values. For thepurposes of. the present invention, the resistance 55 is preferably ofthe greater value so that current flow in the discharge space tends tomaintain the grid 31 negative, with respect to the grid 38.

.The resulting establishment of a definite field between the grids 38and 31 favors the creation of a completely closed ion sheath around thelatter and thus increases its effectiveness in preventing a discharge.

The functioning of the grid 33 in this manner is due to the fact that itdefines on the cathode side of grid 31 a segregated space from whichionization can disappear rapidly after cessation of conduction. In thisway it serves to hasten the creation of a discharge-preventing, closedion sheath around the grid 31.

In connection with the apparatus as so far described it will be seenthat if the grid 31 is made equipotential with or negative to thecathode at a time when a high positive potential is applied to the anodeI3, the entire anode-cathode voltage will appear between the anode andgrid. If it is desired to use the apparatus at voltages as high as orabove 100,000 volts, this consideration makes the avoidance of prematuredischarge a matter of some difliculty. Where such breakdown occurs, itis apt to be due to uneven distribution of the potential stress and theconsequent occurrence of excessive gradients along. some portion of thedischarge path. In order to improve this situation, my invention employsmeans for subdividing the potential drop between the grid 31 and theanode H). To this end, the grid 36 is connected through a resistance 58to the midpoint of a potential divider, exemplified by a resistance 59.With this arrangement the potential of the grid 33 is fixed at a valuemidway between the potentials of the cathode and anode and approximatelymidwaybetween the potentials of the anode and the grid 31.

The successful functioning of the grid 36 as a voltage divider dependsat least in part upon the substantial absence of ionization in thedischarge space. To the extent that such ionization exists, there is atendency for the grid 36 to be unable to prevent the main anode fieldfrom extending to the grid 31 and thus concentrating the potentialstress across the ion sheath which surrounds that grid. My presentinvention includes as another important feature thereof the ues of meansfor lessening the completeness of deionization required to permiteffective subdivision of the interelectrode potential stress. Theprovision of such means has for its ultimate object the possibility ofdecreasing the time interval which must elapse between the end of oneconducting period of the apparatus and the instant when adischarge-favoring potential may again be applied to the apparatuswithout the immediate occurrence of breakdown.

In the arrangement shown, the means referred to in the foregoingparagraph comprises the grid 35 and energizing connections therefor.Such connections include a transformer 62 which is shown as beingenergized from the same voltage source as the transformer 5| and whichis so connected as to make the grid 35 negative with respect to the grid36 whenever the grid 31 becomes negative with respect to the cathode 23.A resistor 63 is connected in circuit with the secondary of thetransformer 62 to limit the current flow therethrough and a condenser 64is connected across the transformer to diminish the effect of transientinfluences.

The occurrence of a definite potential field between the grids 35 and 35during a period when residual ionization still exists in the dischargespace appears to facilitate the creation of a closed positive ion sheathacross the grid 35 and thus to hasten the moment when that grid iseffective to prevent the penetration of the anode field into the spacebelow it. When this condition is attained, the voltage in the dischargespace is efiectively divided between the region associated with the grid35 and that associated with the grid 31. Furthermore, this condition isattained at a period appreciably earlier than would be possible withoutthe provision of the elements referred to.

The principle of subdivision of stress may be carried to any desiredlength by providing additional grid pairs between the anode and the grid31. The individual members of such grid pairs should, of course, beinterrelated in the manner of the electrodes 35 and 36, andappropriately spaced connections to the potential divider 59 should beprovided. In this latter connection, it will be understood that it isimmaterial which member of the grid pair is connected to the divider aslong as the point of connection is properly selected.

The invention described in the foregoing is of special utility inconnection with apparatus desired to be employed for inversion purposes;that is to say, for the conversion of direct current to alternatingcurrent. A circuit illustrating this application is shown in Fig. 3 andserves further to explain the nature and utility of the invention.

Referring to Fig. 3 there are shown two sets of apparatus of thecharacter described in Figs. 1 and 2, these sets being arranged inback-to-back relationship and being associated with additional circuitmeans adapted to cause them to function conjointly as an inverter. Inorder to simplify the explanation, elements corresponding to partsalready described have been similarly numbered, and the numerals appliedto the apparatus at the left-hand part of the drawing have beendiiIerentiated from those applied to the right-hand apparatus by theexpedient of priming the numbers.

In the circuit illustrated, the anodes l9 and I9 are connected with therespective terminals of a transformer winding 61. The midpoint of,

this winding and the cathodes 23 and 28' of the respective dischargetubes are connected to the terminals of a source of direct currentpotential as indicated at 68. The winding 61 is coupled to a secondarywinding 69 which in turn is connected to an alternating current loaddevice .10.

Winding 61 is further shunted by means of a condenser I2 which serves tofacilitate commutation of the discharge between the two discharge tubesin a manner to be more fully explained in the following.

Inverter circuits of the general character of that described above arewell known to the art so that their operation need not be elaboratedherein. It may be stated generally that a conduction period through anyone discharge tube is eventually terminated by the initiation ofdischarge through the other tube. At the instant the discharge through agiven tube ceases, the commutating condenser (e. g. condenser I2 of Fig.3) drives the anode potential of such tube negative for a brief intervalof time to permit deionization to occur. The deionizing period must bemade sufficiently long so that when positive potential is again appliedto the anode, premature and undesired breakdown will not occur. Thelonger the interval required for this purpose, the larger must be thecapacity of the commutating condenser, so that for obvious reasons ofeconomy it is desirable to make the necessary deionizing period as shortas possible.

In order that the apparatus of Fig. 3 may serve the purpose referred toin the preceding paragraph, the excitation of the transformers 5| and 5|is synchronized with the voltage developed across the load device tomaintain a desired and definite phase relationship between thepotentials applied to the various grids and those applied between therespective anodes and cathodes. This may be done, for example, byconnecting the transformers 5| and 51' to the terminals of thetransformer winding 69 through a phase shifter (not shown) or othermeans adapted to control phase relationship.

For the apparatus to operate in the intended fashion, it is desirablethat grid shall be rendered negative with respect to grid 36 and thatgrid 37 shall be rendered negative with respect to cathode 23 at, orshortly before the time when positive potential is impressed on theanode l9. (Similarly, of course, the grids 35' and 31 should be renderednegative before positive potential is applied to anode l 9'.) The timeinterval by which the latter occurrence, namely, the application ofpositive potential to the anode elements, must be delayed is appreciablydecreased by the use of the grid combinations 35, 31 and 35', 31'; thisbeing for the reason that the presence of these grids serves tofacilitate the attainment of efiective grid control even though theprocess of deionization may be relatively far from completion. Thus theuse of this aspect of my invention serves to reduce the necessary sizeof the condenser 12 and thereby to diminish the cost of the inverter asa whole.

While I have described my invention in connection with a particularembodiment thereof, it will be understood that numerous modificationsmay be made therein by those skilled in the art without departing fromthe invention. I therefore, aim in the appended claims to cover all suchvariations as come within the true spirit and scope of the foregoingdisclosure.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. In discharge apparatus of the type in which the discharge takes placethrough an ionizable medium, the combination which includes an anode, acathode, a control grid between the anode and cathode, means forcontrolling the potential of the said grid in a manner tending toprevent the occurrence of a discharge between the anode and cathodeduring predetermined intervals, and means for fixing the potentialdistribution between the grid and the anode, said means including a pairof intermediate electrodes, circuit connections for fixing therelationship between the potential of one electrode of said pair and thepotentials of the anode and cathode, and means for rendering thepotential of the one of said electrodes which is nearer to the anodenegative with respect to the other of said electrodes, at least during aportion of the said predetermined intervals.

2. High voltage apparatus comprising a discharge device having an anode,a cathode, an ionizable operating medium and at least three gridelectrodes successively interposed between the anode and cathode, meansfor applying a discharge-producing potential between the anode andcathode, means for applying a control potential to the one of said threegrid electrodes which is nearest the cathode, means for fixing therelationship between the potential of one of the other two gridelectrodes and the potentials of the anode and cathode, and means efiective during periods of desired non-conduction of one of said threeelectrodes which is nearest to the anode negative with respect to thecentral one of such electrodes.

3. In a discharge apparatus of the type in which conduction takes placethrough an ionizable medium, the combination which includes a dischargedevice having an anode and cathode, an energizing circuit connectingwith said anode and cathode, the circuit being of such character thattermination of one conducting period of the device is followed almostimmediately by the application between the anode and cathode of apotential favoring the renewal of conduction, a control grid between theanode and cathode, means for applying to the control grid a cyclicallyvariable potential so phased as to inhibit renewal of conduction by thedevice during predetermined intervals, and means for fixing thepotential distribution between the control grid and the-anode tofacilitate the desired functioning of the former, said last-named meansincluding a pair ofintermediate electrodes, circuit connections forfixing the relationship between the potential of one of said electrodesand the potentials of the anode and cathode, and means effective atleast during the said predetermined intervals to render the one of saidelectrodes nearer the anode negative with respect to the otherelectrode.

4. In a discharge apparatus of the type in which conduction takes placethrough an ionizable medium, the combination which includes a dischargedevice having an anode and cathode, an energizing circuit connectingwith said anode and cathode, the circuit being of such character thatterthe desired functioning of the former, said lastnamed means includinga pair of intermediate electrodes, 8. potential divider for fixing therelationship between the potential of one of said electrodes and thepotentials of the anode and cath- 5 ode, and a source of cyclicallyvariable voltage for rendering the one of said electrodes nearer theanode negative with respect to the other during the said predeterminedintervals and positive with respect to such other electrode duringperiodsof 1 desired conduction of the discharge device. I

KENNETH H. KINGDON.

